Multi-stage screening apparatus screen basket and method for screening pulp suspensions

ABSTRACT

Screening apparatus for screening pulp suspensions is disclosed comprising a screen basket dividing the interior of the housing into a central chamber and a single outer annular chamber. The suspension is supplied to one of the outer and central chambers, so that an accept fraction passes through the screen, while a reject fraction develops that is prevented from passing through the screen. Dilution is provided by supplying diluting liquid to dilute the reject fraction. The screen basket is divided into at least two separate tubular screen sections. The dilution is provided by at least one annular element axially interconnecting the two tubular screen sections and forming a tubular dilution liquid compartment extending at least substantially around the screen basket. The annular element forms a plurality of dilution liquid ejection passages between the dilution liquid compartment and one of the outer and central chambers.

FIELD OF THE INVENTION

The present invention relates to a screening apparatus for screeningpulp suspensions, comprising a housing, a tubular screen basket dividingthe interior of the housing into a central chamber and an outersubstantially annular chamber, an inlet member for supplying asuspension to be screened into one of the central chamber and outerchamber, and an accept outlet member for discharging a developed acceptfraction of the suspension that has passed through the screen basket.The apparatus further comprises, a reject outlet member for discharginga developed reject fraction of the suspension, a rotor arranged in thehousing for providing pressure and suction pulses in the suspension tobe screened along the screen basket, and dilution means for supplyingdiluting liquid to one of the central chamber and outer chamber.

The present invention also relates to a screen basket for use in such anapparatus and methods of screening pulp suspensions in several screeningstages.

BACKGROUND OF THE INVENTION

A very important step in the papermaking process is screening of fiberpulp suspensions. Traditionally, the pulp suspension is screened byseveral so-called pressure screening apparatus of the type describedabove interconnected in a system of screening apparatuses, in which eachscreening apparatus represents a screening stage dependent on the otherstages of the system.

As an alternative to the traditional screening system with severalinterconnected screening apparatus, one single screening apparatus maybe designed with several stages, typically two or three stages,incorporated into the same screen body. A variety of such multi-stagescreening apparatus of various designs have recently been introduced tothe market.

The increasing size of the paper making production lines of today hasresulted in very large screening apparatus. Especially screeningapparatus for low consistency pulp suspensions is large and has a verylarge screen basket, in order to accommodate high hydraulic loads. Thescreen baskets for different screening apparatus are typically designedwith about the same aspect ratio—length/diameter—regardless of size, sothat a large basket is very long. Another reason why many screen basketsare long is the fact that it is considerably cheaper to increase thesize of a given screen by increasing the length of the screen basket ascompared to increasing the diameter thereof.

In a long screen basket the path of travel for debris particles will belong. As a consequence, a long screen basket has the disadvantage thatsince the retention time for the individual particle that is to berejected will be long the probability of acceptance or breakdown will behigher than in shorter screen baskets. Furthermore, a long screen basketis likely to encounter problems with reject fraction thickening and willhave lower capacity per unit surface area as well as reduced removalefficiency.

One way to counteract the reject fraction thickening is to dilute itwith dilution liquid, typically water, and there are prior screeningapparatus provided with arrangements to add dilution water to the insideof the screen basket for this purpose. For example, U.S. Pat. Nos.6,080,274 and 6,186,333, and WO 00/50690 disclose expensive dilutionwater arrangements built into multi-stage screening apparatus. A seriousdisadvantage of these known multistage screening apparatus is the needfor expensive hardware for process control in form of very large valvesand flow meters on the accept lines from the different stages. Eachaccept compartment requires a separate flow control with flow meters andcontrol valves.

Another known dilution arrangement includes revolving dilution wateroutlets integrated into the rotor. However, with this kind of dilutionarrangement it is difficult to get the pressurised dilution water fromthe screen housing into the rotor. There are seals between stationaryand rotary parts of the apparatus that often have wear problems, so thatfibers pass through the seals into the dilution water compartments andeventually plug the outlets for dilution water. Another known dilutionarrangement includes stationary dilution water outlets below thescreening zone, and integrated into the screen housing. With thesefairly expensive known arrangements it is very difficult to transportthe dilution water to the optimum destination in the screen basket.

Swedish patent application No. 9601979-9 proposes a solution to theabove noted problems and discloses a dilution arrangement in whichdilution water is introduced into a channel circumventing a wedge wiretype of screen basket. The channel is formed by putting a lid over thespace between two support rings on the screen basket. The dilution wateris fed into the screen basket through screening slots provided on themantle wall of the screen basket. However, a problem with this solutionis that the flow of dilution water entering the inside of the screenbasket through the many very fine screen slots is insufficient andcannot provide for enough penetration and mixing of the dilution waterand the thickened reject fraction. Another problem is leakage of unknownquantities of water to the accept chamber located external to the screenbasket through the axially open spaces at the outer narrow ends betweenthe wedge shaped bars and the fixation and support rings, thatconstitute the top and bottom of the dilution water channel.

One object of the present invention is to provide a screening apparatusfor screening pulp suspension in stages having a simple, inexpensivedilution means that supplies dilution liquid to an optimum destinationin the screen basket for efficient dilution of the reject fraction.

Another object of the invention is to provide a screen basket for use inthe screening apparatus of the present invention and also for replacingworn out screen baskets in existing screening apparatuses.

Yet another object of the present invention is to provide a method ofscreening pulp suspension in stages so that the developed rejectfraction is diluted in an optimum manner.

The initial object set forth above is obtained by a screening apparatusof the type described initially characterized in that the screen basketincludes at least two separate tubular screen sections, and that thedilution means comprises at least one annular element axiallyinterconnecting the two tubular screen sections and forming a tubulardilution liquid compartment extending at least substantially around thescreen basket, the annular element forming a plurality of dilutionliquid ejection passages between the dilution liquid compartment and oneof the central chamber and outer chamber.

As a result, the required amount and velocity of the dilution liquidjets sprayed from the ejection passages to provide efficient dilution ofthe reject fraction is easy to achieve by properly designing the size ofthe ejection passages.

SUMMARY OF THE INVENTION

In accordance with an embodiment of the present invention, one of theaccept outlet member and reject outlet member, normally the acceptoutlet member, forms an outlet passage from the outer chamber, and thedilution means comprises at least one dilution liquid supply conduitextending through the outlet passage to the annular element, to supplydilution liquid from outside the housing to the dilution liquidcompartment. This embodiment enables easy and inexpensive connection ofthe dilution liquid supply conduit with the annular element, becausethere is no need for any separate connection through the housing. Theoutlet member may include a releasable outlet portion situated outsidethe housing, wherein the dilution liquid supply conduit extends throughthe wall of the releasable outlet portion.

In accordance with another embodiment of the present invention, thedilution means comprises first and second dilution liquid supplyconduits connected to the annular element at different places thereon.This embodiment provides a more even distribution of dilution liquidinto the screen basket.

In both of these embodiments the tubular dilution liquid compartment mayextend in a closed loop around the screen basket, and the dilutionliquid supply conduit may be arranged to direct the dilution liquid intothe dilution liquid compartment such that the dilution liquid flows inone direction along said closed loop. As a result, the flow of dilutionliquid circling in the dilution liquid compartment will counteractfibers that might enter the compartment from depositing on thecompartment wall.

Another of the objects of the present invention is obtained by a screenbasket, which comprises a tubular mantle wall provided with screenholes, and dilution means for supplying dilution liquid to one of theinside and outside of the tubular mantle wall. The screen basket ischaracterised in that the tubular mantle wall includes at least twoseparate tubular wall sections, and the dilution means comprises atleast one annular element axially interconnecting the two tubular wallsections of the mantle wall and forming a tubular dilution liquidcompartment extending at least substantially around the tubular mantlewall, the annular element forming a plurality of dilution liquidejection passages between the dilution liquid compartment and one of theinside and outside of the screen basket.

An important advantage of the screen basket of the present invention isthat it is well suited for replacing worn out screen baskets in existingsingle-stage screening apparatuses, thereby functionally converting theexisting apparatuses into multi-stage apparatuses.

The ejection passages may have circular cross-sections or,alternatively, take the shape of slots.

Suitably, the tubular dilution liquid compartment has a rectangularcross-section and extends in a closed loop around the tubular mantlewall.

The dilution means may comprise first and second dilution liquid supplyinlets on the annular element positioned at different places thereon.

Another of the objects of the present invention is obtained by a methodof screening a pulp suspension by the use of a screening apparatushaving a tubular screen basket. The method comprises:

-   -   feeding the suspension to be screened to one of the external        side and internal side of the screen basket,    -   screening the suspension along a primary screening section of        the screen basket to obtain a primary accept fraction that        passes through the screen basket and a primary reject fraction        that is prevented from passing through the screen basket,    -   supplying a flow of dilution liquid to dilute the primary reject        fraction,    -   screening the diluted primary reject fraction along a secondary        screening section of the screen basket to obtain a secondary        accept fraction that passes through the screen basket and a        secondary reject fraction that is prevented from passing through        the screen basket,    -   discharging the secondary reject fraction from the screen        basket, and    -   combining the primary and secondary accept fractions to form a        common final accept fraction.

The method is characterized by:

-   -   controlling the flow of dilution liquid being supplied in        response to the consistency and flow of the suspension being fed        to the screen basket and the consistency and flow of the        secondary reject fraction being discharged from the screen        basket, so that the consistency of the primary reject fraction        entering the secondary screening section becomes substantially        the same as the consistency of the suspension being fed to the        screen basket.

The suspension to be screened is normally fed into the internal side ofthe screen basket and is screened so that the primary reject fractiondevelops inside the screen basket, whereby the flow of dilution liquidis supplied to the inside of the screen basket and the secondary rejectfraction develops inside the screen basket.

The method may further comprise supplying the flow of dilution liquid inthe form of jets having a velocity in the range of 2-10 m/s, preferably4-8 m/s.

The control of the flow of dilution liquid being supplied to the screenbasket is based on an algorithm calculated as follows.

Thickening is the result of that the probability for acceptance throughthe screen barrier always is higher for water than for fiber. It isdefined as the consistency increase from the feed end to the reject endof the screen basket. Thickening varies with the type of pulp, theproduction rate and with most design and operating variables of apressure screen.

The thickening is the ratio F between reject consistency Cr and feedconsistency Cf or the ratio between mass reject rate Rm and volumetricreject rate Rv. $\begin{matrix}{F = {\frac{Cr}{Cf} = \frac{Rm}{Rv}}} & (1)\end{matrix}$with the assumptions that the thickening in the two screening stages ina two-stage system are the same and that the mass rejects rate are thesame in the two stages it is possible to calculate the required amountof dilution water Qd. A prerequisite for this calculation is that thevolume flow and mass consistency of the feed and the reject flows areknown. From these assumptions it is possible to derive the followingequation for the required amount of dilution water: $\begin{matrix}{{Qd} = {\sqrt{{Qf}*{Qr}}\left( {\sqrt{\frac{Cr}{Cf} -}\sqrt{\frac{Cf}{Cr}}} \right)}} & (2)\end{matrix}$where Qd is the amount of dilution liquid, Qf is the volume flow of thefeed, Qr is the volume flow of the reject fraction, Cf is theconsistency (mass concentration) of the feed and Cr is the consistencyof the reject fraction.

This is the algorithm that makes it possible to adjust and control theamount of dilution water so that the feed consistency, to the secondaryscreening stage of screen basket, will become the same as that of theprimary stage. The input data required for this calculation is onlyfeed—and final reject flow and the consistencies of these flows.

The control algorithm can also be writtenQd=√{square root over (Qf*Qr)}(√{square root over (F)}−√{square rootover (F ⁻¹)})  3

As an alternative to the above method of the present invention, whichrelates to two-stage screening, one of the objects of the presentinvention also is obtained by a method for three-stage screening.Accordingly, the alternative method comprises:

-   -   feeding the suspension to be screened to one of the external        side and internal side of the screen basket,    -   screening the suspension along a primary screening section of        the screen basket to obtain a primary accept fraction that        passes through the screen basket and a primary reject fraction        that is prevented from passing through the screen basket,    -   supplying a first flow of dilution liquid to dilute the primary        reject fraction,    -   screening the diluted primary reject fraction along a secondary        screening section of the screen basket to obtain a secondary        accept fraction that passes through the screen basket and a        secondary reject fraction that is prevented from passing through        the screen basket,    -   supplying a second flow of dilution liquid to dilute the        secondary reject fraction,    -   screening the diluted secondary reject fraction along a tertiary        screening section of the screen basket to obtain a tertiary        accept fraction that passes through the screen basket and a        tertiary reject fraction that is prevented from passing through        the screen basket,    -   discharging the tertiary reject fraction from the screen basket,    -   combining the primary, secondary and tertiary accept fractions        to form a common final accept fraction,

The alternative method is characterized by:

-   -   controlling the first and second, respectively, flow of dilution        liquid being supplied to the screen basket in response to the        consistency and flow of the suspension being fed to the screen        basket and the consistency and flow of the tertiary reject        fraction being discharged from the screen basket, so that the        consistency of the primary reject fraction entering the        secondary screening section and the consistency of the secondary        reject fraction entering the tertiary screening section,        respectively, becomes substantially the same as the consistency        of the suspension being fed to the screen basket.

As mentioned above the suspension to be screened is normally fed intothe internal side of the screen basket. Thus, the suspension is screenedso that the primary reject fraction develops inside the screen basket,whereby the first and second flows of dilution liquid are supplied tothe inside of the screen basket and the secondary and tertiary rejectfractions develop inside the screen basket

It is possible to derive a similar equation for the amount of dilutionwater required in the first Qd₁ and the second Qd₂ dilution water stage.The derivation of the formulas is similar to the case with two stagesshown above.

The amount of dilution water required after the first stage of screeningto obtain the same feed consistency to the second stage of screening asthe feed to the screen, the first stage of screening can be calculatedby the following formula:Qd ₁={cube root}{square root over (Qf ² *Qr)}({cube root}{square rootover (F)}−{cube root}{square root over (F ⁻²)})  4

The general formula for two and three stage applications isQd ₁=^(n)√{square root over (Qf ^(n−1) Qr)}(^(n)√{square root over(F)}−^(n)√{square root over (F ^(1−n))})  5where the number of screening stages is (n)

Under the same assumptions the required amount to the second dilutionstage in three-stage screen basket isQd ₂ =Qd ₁{cube root}{square root over (Rm)}  6

-   -   where (Rm) is the total mass reject rate over the whole screen,        after the three stages. Retrofits for all types of screens with        long baskets, e.g. all screen baskets longer (higher) than 600        mm will benefit from the present invention. They will have        increased capacity and/or efficiency by dividing a too long        screening zone into a primary and a secondary stage. These        positive effects are results of a more efficient utilization of        the screen basket surface.

Another possibility to take advantage of this new concept is to operatewith a more gentle surface profile of the screen basket that defines thescreen holes for better removal efficiency. A too aggressive surfaceprofile is not required to meet capacity demands. Multi-stage dilutionwill also make it possible to reduce the RPM of the rotor. At lower RPMthe screening apparatus will pull lower electric load.

New product lines of screens can take advantage of this technology. Nodilution arrangements will be required in the screen housing and/or inthe screen rotor. A simpler and less expensive screen design can beused.

The multi-stage screening apparatus of the invention can be designedwith only one accept compartment and with less expensive processcontrol. For example, a two-stage screen with controlled mass rejectrate based on state of the art technology requires four flow controlsand two consistency controllers, whereas the multi-stage dilutiontechnology according to the present invention requires only three andtwo controls respectively for the same information.

The same comparison for a three-stage apparatus is even moreadvantageous in favor of the present invention. The additional stagerequires only one more flow controller. With conventional technology twomore controllers would be needed.

If the multi-stage dilution technology is combined with control of thescreen rotor RPM it will be possible to obtain maximum removalefficiency of a “two stage system” under very varying processconditions.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is described in more detail in the followingdetailed description with reference to the accompanying drawings, inwhich

FIG. 1 is a side, perspective, partial cut-away view of a firstembodiment of the screening apparatus of the present invention,

FIG. 2 is a side, perspective, partial cut-away view of a secondembodiment of the invention,

FIG. 3 is a side, perspective view of a screen basket that fits theapparatus according to FIG. 1, and

FIG. 4 is a side, sectional perspective view of the screen basket shownin FIG. 3.

DETAILED DESCRIPTION

Identical components shown in the figures are denoted with the samereference numerals.

FIG. 1 shows a screening apparatus according to the present inventionfor screening pulp suspensions, comprising a housing 2, an inlet member4 releasably connected to a supply pipe 6 for supplying a suspension tobe screened into the housing 2, a tubular screen basket 8 dividing theinterior of the housing 2 into a central substantially cylindricalchamber 10 for receiving the suspension to be screened at one end 12 ofthe central chamber and a single outer annular accept chamber 14 forreceiving an accept fraction of the suspension that has passed throughthe screen basket 8, an accept outlet member 16 forming an outletpassage 17 and releasably connected to an accept outlet pipe 18 fordischarging the accept fraction from the accept chamber 14 and a rejectoutlet member 20 releasably connected to a reject outlet pipe 22 fordischarging a reject fraction of the suspension from the central chamber10 at the other end 24 thereof. A rotor 26 is arranged in the centralchamber 10 for providing pressure and suction pulses in the suspensionalong the internal side of the screen basket 8. Dilution means 28 isprovided for supplying diluting liquid to the central chamber 10 betweenthe ends 12 and 24 thereof.

The screen basket 8 comprises a cylindrical mantle wall 30 with screenholes taking the shape of slots. The mantle wall 30 is provided with anupper flange 32 and a lower flange 34 that seal against an uppershoulder 36 on the housing and a lower shoulder 38 on the housing,respectively. With reference to FIGS. 3 and 4, the mantle wall 30 isdivided into two separate cylindrical sections 40 and 42, which areaxially interconnected by an annular element 44 of the dilution means28. The annular element 44 forms a tubular dilution liquid compartment46 having a rectangular cross-section and extending around the mantlewall 30. The annular element 44 has a dilution liquid inlet opening 48and a multiplicity of dilution liquid ejection passages 50 havingcircular cross-section and extending between the compartment 46 and theinside of the screen basket 8. A dilution liquid supply conduit 52 ofthe dilution means 28 for supplying dilution liquid from outside thehousing 2 to the dilution liquid compartment 46 extends through the wallof the accept outlet pipe 18 and further through the outlet passage 17of the accept outlet member 16 to the opening 48 of the annular element44.

The screen basket 8 described above is particularly suited for replacingtraditional single stage screen baskets in old screening apparatuses. Byutilizing the existing accept outlet member to connect the dilutionliquid supply conduit there is no need for reconstructing the housing ofthe old apparatus.

In operation, a fiber suspension to be screened is fed via the inletmember 4 to the screen basket 8 at the upper side 12 thereof. In thescreen basket 8 the suspension is screened along section 40 of themantle wall 30, so that a primary accept fraction passes through themantle wall 30 while a primary reject fraction develops inside thescreen basket 8. The primary reject fraction is diluted by a controlledflow of dilution liquid sprayed through the ejection passages 50. Thediluted primary reject fraction is screened along section 42 of themantle wall 30, so that a secondary accept fraction passes through themantle wall 30 while a secondary reject fraction develops inside thescreen basket 8 and then is discharged from the screen basket 8 throughthe reject outlet member 20. The primary and secondary accept fractionsare combined and discharged through the accept outlet member 16.

The flow of dilution liquid through the ejection passages 50 iscontrolled in response to the consistency and flow of the suspensionbeing fed to the screen basket 8 and the consistency and flow of thesecondary reject fraction being discharged from the screen basket 8, sothat the consistency of the primary reject fraction entering section 42of the mantle wall 30 becomes substantially the same as the consistencyof the suspension being fed into the screen basket 8.

The above-described embodiment of the present invention according toFIG. 1 is of a type most commonly used. However, in an alternativeembodiment of the invention, not shown, the suspension is supplied tothe outer chamber 14 and a rotor is arranged in the outer chamber 14 toprovide pressure and suction pulses in the suspension along the externalside of the screen basket 8. In this alternative embodiment, the liquidejection passages extend between the compartment 46 and the outside ofthe screen basket 8, so that the primary reject fraction that developsoutside the screen basket 8 can be diluted by liquid jets from theejection passages.

FIG. 2 shows a screening apparatus of the present invention similar tothe embodiment shown in FIG. 1 except that the screen basket and thedilution liquid supply are designed differently. Thus, the apparatus ofFIG. 2 comprises a housing 54 provided with two dilution liquid inletconduits 56 and 58, and a screen basket 60 provided with two dilutionliquid inlet openings 62 and 64 connected to the conduits 56 and 58,respectively. This embodiment is suited for new screening apparatuses.

Although the invention herein has been described with reference toparticular embodiments, it is to be understood that these embodimentsare merely illustrative of the principles and applications of thepresent invention. It is therefore to be understood that numerousmodifications may be made to the illustrative embodiments and that otherarrangements may be devised without departing from the spirit and scopeof the present invention as defined by the appended claims.

1. A screening apparatus for screening pulp suspensions, comprising: ahousing, a tubular screen basket dividing the interior of the housinginto a central chamber and an outer substantially annular chamber, thescreen basket including at least two separate tubular screen sections,an inlet member for supplying a suspension to be screened into one ofthe central chamber and outer chamber, an accept outlet member fordischarging a developed accept fraction of the suspension that haspassed through the screen basket, a reject outlet member for discharginga developed reject fraction of the suspension, a rotor arranged in thehousing for providing pressure and suction pulses in the suspension tobe screened along the screen basket, and dilution means for supplyingdiluting liquid to one of the central chamber or outer chamber, thedilution means including at least one annular element axiallyinterconnecting the two tubular screen sections and forming a tubulardilution liquid compartment extending at least substantially around thescreen basket, the annular element forming a plurality of dilutionliquid ejection passages between the dilution liquid compartment and oneof the central chamber and outer chamber.
 2. A screening apparatusaccording to claim 1, wherein one of the accept outlet member and rejectoutlet member forms an outlet passage from the outer chamber, and thedilution means comprises at least one dilution liquid supply conduitextending through the outlet passage to the annular element, to supplydilution liquid from outside the housing to the dilution liquidcompartment.
 3. A screening apparatus according to claim 2, furthercomprising an outlet pipe releasably connected to the outlet member thatforms said outlet passage, wherein the dilution liquid supply conduitextends from outside the housing through the wall of the outlet pipeinto the outlet passage.
 4. A screening apparatus according to claim 1,wherein the dilution means comprises first and second dilution liquidsupply conduits connected to the annular element at different placesthereon.
 5. A screening apparatus according to claim 1, wherein thetubular dilution liquid compartment extends in a closed loop around thecentral chamber, and the dilution liquid supply conduit is arranged todirect the dilution liquid into the dilution liquid compartment suchthat the dilution liquid flows in one direction along said closed loop.6. A screening apparatus according to claim 4, wherein the tubulardilution liquid compartment extends in a closed loop around the centralchamber, and the first and second dilution liquid supply conduits arearranged to direct the dilution liquid into the dilution liquidcompartment such that the dilution liquid flows in one direction alongsaid closed loop.
 7. A screen basket comprising. a tubular mantle wallprovided with screen holes and including at least two separate tubularwall sections, and dilution means for supplying dilution liquid to oneof the inside and outside of the tubular mantle wall, the dilution meansincluding at least one annular element axially interconnecting the twotubular wall sections of the mantle wall and forming a tubular dilutionliquid compartment extending at least substantially around the tubularmantle wall, wherein the annular element forms a plurality of dilutionliquid ejection passages between the dilution liquid compartment and oneof the inside and outside of the screen basket.
 8. A screen basketaccording to claim 7, wherein the ejection passages have circularcross-sections.
 9. A screen basket according to claim 7, wherein theejection passages take the shape of slots.
 10. A screen basket accordingto claim 7, wherein the tubular dilution liquid compartment has arectangular cross-section.
 11. A screen basket according to claims 7,wherein the dilution means comprises first and second dilution liquidsupply inlets on the annular element positioned at different placesthereon.
 12. A screen basket according to claims 7, wherein the tubulardilution liquid compartment extends in a closed loop around the tubularmantle wall.
 13. A method of screening a pulp suspension by the use of ascreening apparatus having a tubular screen basket, the methodcomprises: feeding the suspension to be screened to one of the externalside and internal side of the screen basket, screening the suspensionalong a primary screening section of the screen basket to obtain aprimary accept fraction that passes through the screen basket and aprimary reject fraction that is prevented from passing through thescreen basket, supplying a flow of dilution liquid to dilute the primaryreject fraction, screening the diluted primary reject fraction along asecondary screening section of the screen basket to obtain a secondaryaccept fraction that passes through the screen basket and a secondaryreject fraction that is prevented from passing through the screenbasket, discharging the secondary reject fraction from the screenbasket, combining the primary and secondary accept fractions to form acommon final accept fraction, and controlling the flow of dilutionliquid being supplied in response to the consistency and flow of thesuspension being fed to the screen basket and the consistency and flowof the secondary reject fraction being discharged from the screenbasket, so that the consistency of the primary reject fraction enteringthe secondary screening section becomes substantially the same as theconsistency of the suspension being fed to the screen basket.
 14. Amethod according to claim 13, wherein the suspension to be screened isfed into the internal side of the screen basket and is screened so thatthe primary reject fraction develops inside the screen basket, wherebythe flow of dilution liquid is supplied to the inside of the screenbasket and the secondary reject fraction develops inside the screenbasket.
 15. A method according to claim 13 or 14, further comprisingsupplying the flow of dilution liquid in the form of jets having avelocity in the range of 2-10 m/s, preferably 4-8 m/s.
 16. A method ofscreening a pulp suspension by the use of a screening apparatus having atubular screen basket with perforations, the method comprises: feedingthe suspension to be screened to one of the external side and internalside of the screen basket, screening the suspension along a primaryscreening section of the screen basket to obtain a primary acceptfraction that passes through the screen basket and a primary rejectfraction that is prevented from passing through the screen basket,supplying a first flow of dilution liquid to dilute the primary rejectfraction, screening the diluted primary reject fraction along asecondary screening section of the screen basket to obtain a secondaryaccept fraction that passes through the screen basket and a secondaryreject fraction that is prevented from passing through the screenbasket, supplying a second flow of dilution liquid to dilute thesecondary reject fraction, screening the diluted secondary rejectfraction along a tertiary screening section of the screen basket toobtain a tertiary accept fraction that passes through the screen basketand a tertiary reject fraction that is prevented from passing throughthe screen basket, discharging the tertiary reject fraction from thescreen basket, combining the primary, secondary and tertiary acceptfractions to form a common final accept fraction, and controlling thefirst and second, respectively, flow of dilution liquid being suppliedto the screen basket in response to the consistency and flow of thesuspension being fed to the screen basket and the consistency and flowof the tertiary reject fraction being discharged from the screen basket,so that the consistency of the primary reject fraction entering thesecondary screening section and the consistency of the secondary rejectfraction entering the tertiary screening section, respectively, becomessubstantially the same as the consistency of the suspension being fed tothe screen basket.
 17. A method according to claim 16, wherein thesuspension to be screened is fed into the internal side of the screenbasket and is screened so that the primary reject fraction developsinside the screen basket, whereby the first and second flows of dilutionliquid are supplied to the inside of the screen basket and the secondaryand tertiary reject fractions develop inside the screen basket.
 18. Amethod according to claim 16, further comprising supplying the first andsecond flows of dilution liquid in the form of jets having a velocity inthe range of 2-10 m/s, preferably 4-8 m/s.